1. Field of the Invention
The present invention relates to hard disk drives. More particularly, the present invention relates to a method for reassigning a defective data site on a disk surface during a write operation.
2. Description of the Prior Art
Hard disk drives store large volumes of data on one or more disks mounted on a spindle assembly. Disk drives employ a disk control system for interfacing with a host (e.g., a computer) to control the reading and writing of data on a disk. Each disk includes at least one disk surface which is capable of storing data. On each disk surface, user data is stored in concentric circular tracks between an outside diameter and an inside diameter of the disk.
As a result of the manufacturing process, defective data sites may exist on the disk surfaces of the disk drive. These defective data sites are termed "primary defects". A defect discovery procedure is performed to locate these defects and mark them out as defective locations on the disk surface which are not available for use. A typical defect discovery procedure includes writing a known data pattern to the disk surface and subsequently reading the data pattern from the disk surface. Defective data sites are identified by comparing the data pattern read from the disk surface with the known data pattern written to the disk surface.
Following the defect discovery procedure, defective data sites are put in a primary defect list which is stored in a table. The primary defect list is used during formatting of the disk surface to generate a defect management table. Within the defect management table, the defective data sites may be mapped to data sector locations (cylinder number, head number, and data sector number). Once identified in the defect management table, the defective data sectors may not be used for storing data.
Defective data sites encountered after formatting the disk surface are known as "grown defects" or "secondary defects". Grown defects often occur in locations adjacent to defective data sites found during defect discovery. Grown defects are also listed in a table, similar to that utilized by the "primary defects". The number of sites marked out on a disk drive as "defective data sites" is used as a measure of the quality of the disk drive. Grown defects encountered during the operation of the disk drive are also added to the defect management table. The defect management table is utilized during the operation of the disk drive to identify defective data sites on the disk surface.
Defects such as "primary defects" and "grown defects" are known as hard sector errors (i.e., unrecoverable sector errors). A hard sector error is essentially permanent in nature, thus the sector cannot be recovered. A disk may also contain transient or "soft" data sector errors. A transient error is defined as an error or defect which clears over a period of time. For example, a transient error may occur due to a thermal asperity on the disk surface. A retry mode may be entered, wherein the command (such as a write) is retried a number of times allowing sufficient time to pass for the transient error to clear. Transient errors are also logged on the drive as they occur.
During operation of the disk drive, data blocks contained within marginally defective sectors (i.e., sectors where the "soft" error rates have reached unacceptable levels, but where the data block is still recoverable) may be reassigned to alternate sectors via various methods. Hicken (U.S. Pat. No. 5,822,142) describes a data sector slipping scheme (i.e., push-down scheme) to reassign data blocks from marginally defective sectors on a disk surface. Bish et al. (U.S. Pat. No. 5,235,585) provides another method for reassigning marginally defective data sectors on a disk to spare data sectors on the disk via a vectoring operation.
In contrast to the data recovery/reassignment of marginally defective data sectors described in the preceding paragraph, when unrecoverable (i.e. hard errors) are encountered during a data write operation to a defective data sector, no method currently exists for an "on-the-fly" reallocation of defective data sectors encountered during a write operation to an alternate sector. In a typical write operation, if a write is attempted to a defective data sector, the drive issues "an unrecoverable write error" message to the user of the drive, and the write operation is terminated. When the user receives such a message, the user may assume the disk drive is no longer usable. Alternatively, the disk drive may perform the write operation to the defective data sector, resulting in subsequent loss of the block of data written to the defective data sector.